1. Field of the Invention
The present invention relates to a polyimide film having stability in mechanical properties and dimension over a broad range of temperatures and to a preparation process for the film.
More particularly, the invention relates to a polyimide film essentially consisting of a thermoplastic polyimide having excellent heat resistance and mechanical properties and to a process for the preparation of said polyimide film by melt-extruding the thermoplastic polyimide into the form of a sheet, followed by uniaxially or biaxially stretching the sheet under specific conditions to cause molecular orientation and then subjecting the resultant film to heat-treatment.
2.Description of the Prior Art
Industrial materials for electric and electronic industries, automotive industry and atomic industry have recently been intended to improve reliability over a long period, miniaturize the products and reduce the weight of products. Consequently, plastic films for use in these products have been required to be excellent in heat-resistance, mechanical properties and dimensional stability.
Thermoplastic polymers having heat resistance, for example, polyphenylene sulfide, polyaryl ketone, polyaryl sulfone, polyallylate, polyester and polycarbonate have conventionally been known and investigated to prepare plastic films in order to meet with the above requirement for heat resistance. However, these films have insufficient heat-resistance and their properties are also unsatisfactory for industrial materials. Hence, the field of application for these films has been restricted.
At present, film materials having the highest heat-resistance designed to meet the above requirement are various polyimide films such as Upilex.TM. (a product of Ube Industries Ltd.) and Kapton.TM. (a product of E.I. Du Pont De Nemours & Co.). Since hot molding of these polyimides is difficult, films are prepared by a so-called solution casting method where a polyamic acid precursor solution is cast, solvent is removed, and the residual film is heat-treated. The method is, however, poor in productivity and high in cost.
Polyimide having recurring structural units represented by the formula (I): ##STR2## has been disclosed as a hot-moldable polyimide in, for example, Japanese Patent Laid-Open Publication SHO 62-205124(1987). However, the film obtained by the disclosed process has lower mechanical strength and becomes soft above its glass transition temperature of about 240.degree. C., and additionally changes from transparent to opaque and becomes brittle at temperatures exceeding 280.degree. C. Thus heat-resistance of the film is unsatisfactory.